Sensors for determining the concentration of gas components in gas mixtures, in particular in gases from internal combustion engines, are known. Such sensors are used to preset a fuel/air mixture for operation of the internal combustion engine on the basis of a determination of the oxygen concentration and/or the concentration of reducing gas components such as HC or CO. A specific operating state can be characterized by using the ratio of the oxygen concentration to the fuel concentration. If there is a stoichiometric excess of fuel (rich range), the amount of oxygen in the exhaust gas will be small in comparison with other components, which are partially uncombusted. In the lean range, where oxygen from air is predominant in the fuel/air mixture, the oxygen concentration in the exhaust gas is accordingly high.
To determine the oxygen concentration in the exhaust gas, there are known lambda sensors which detect a lambda value>1 in a lean range, a lambda value<1 in the rich range and a lambda value=1 in a stoichiometric range. In a known manner, an electrochemical measuring cell of the sensor supplies a detection voltage which is sent to a circuit arrangement. The detection voltage depends on an oxygen concentration difference at the at least two measuring electrodes used. A solid electrolyte body which is conductive for oxygen ions is arranged between the measuring electrodes. The detection voltage increases or decreases according to the oxygen concentration in the exhaust gas.
In addition, there are known sensors which are used to determine the concentration of the reducing gas components.
These sensors are often component specific, i.e., there is a separate sensor for each gas component (H2, HC and CO) to be detected.
Both types of sensor supply either a value for the oxygen concentration or the concentration of the reducing gas components. Thus, they supply values which give only an indirect indication of the status of the lambda value. Thus, a sensor for determining oxygen concentrations can supply a certain absolute oxygen concentration from which it is possible to infer the composition of the fuel/air mixture.
A more precise setting of a control status of an internal combustion engine can be achieved by determining the lambda value directly. It is advantageous here to have a compact sensor capable of handling both sensor functions instead of two separate sensors, one for each gas component. Previous attempts to accommodate multiple sensor functions on one substrate have resulted in a complicated layout or a complicated layer structure which has a greater susceptibility to faults and entails high manufacturing costs.
The reducing gas components in the exhaust gas of internal combustion engines are in a thermodynamic equilibrium with the oxygen. The further away the sensors are arranged from the engine in an exhaust duct of the internal combustion engine, the lower the temperature of the exhaust gas, and thus it is difficult to establish a thermodynamic equilibrium from a kinetic standpoint. It is known that the equilibrium reaction can be catalyzed with transition metals. In particular, catalysts containing platinum, palladium or rhodium have proven suitable. In using these metals as the electrode material for the measuring electrodes of sensors, it has proven especially advantageous to have such a catalytic activity on a measuring electrode which is exposed to the exhaust gas. In this way, the oxygen concentration at this measuring electrode can be kept very low, thus yielding a very high potential difference with respect to another electrode exposed to a reference gas. However, such a measuring electrode which catalyzes the establishment of an equilibrium in the gas mixture (equilibrium electrode) does not permit detection of the concentration of the reducing gas components.
Therefore, it is known that materials which inhibit a catalytic effect of the measuring electrode can be added to the measuring electrode exposed to the gas mixture. Thus, German Patent No. 44 08 361 describes a measuring electrode in which adsorption of oxygen on a surface of the measuring electrode has been made possible by the addition of bismuth, platinum, antimony or lead. Thus, the oxygen concentration, which determines the potential of the measuring electrode, at a ternary boundary of the measuring electrode is kept almost constant in operation of the internal combustion engine at lambda>1. The measuring electrode designed in this way reacts essentially to oxygen and is thus a non-equilibrium electrode or a mixed potential electrode. Such a measuring electrode can thus be used as a reference electrode at lambda>1.
In addition, it is known from German Patent No. 44 08 504 that such a mixed potential electrode can be designed by admixture of gold and/or silver. Due to the admixture of gold and/or silver, the catalytic conversion is inhibited by oxidation of CO and/or HC and reduction of NOx. The high affinity of these metals for the reducing gas components is utilized at the measuring electrode. The oxygen concentration and thus the potential of the mixed potential electrode can be kept almost constant due to adsorption of the reducing gas components at the surface of the mixed potential electrode in operation of the internal combustion engine at lambda<1. In this way, such a measuring electrode can be used as a reference electrode at lambda<1. One disadvantage of these two described embodiments of mixed potential electrodes is that they permit only constant potentials for two extreme positions of the lambda value and thus omit the range at lambda values=1, which is especially interesting for control of the control status of the internal combustion engine. In addition, it is impossible in this way to determine the oxygen concentration on the one hand or the concentration of reducing gas components on the other hand by using one and the same measuring electrode.